The present invention relates to a variable focal length camera capable of changing the focal length of a taking lens between at least two values. More particularly, the invention relates to improvements in focus adjusting devices for setting a lens system of the taking lens at a reference lens position when a taking lens is assembled.
The taking lens of a variable focal length camera is comprised of a main lens system and at least one auxiliary lens system which is inserted into and withdrawn from a photo-taking optical axis. The taking lens has at least two optical lengths selectively used, e.g., a relatively short focal length for wide angle photography and a relatively long focal length for telephotography. In wide angle photography, only the main lens system (front lens group) is present on the photo-taking optical axis, and light from an object passes through the main lens system and impinges upon a photographic film. In telephotography, the auxiliary lens system (rear lens group) is inserted into the photo-taking optical axis, and light from an object passes through the main and auxiliary lens systems and impinges upon a photographic film.
After assembling a taking lens in a camera, the focal point of the lens system of the taking lens is adjusted. According to this focus adjustment, called back focus adjustment, the lens system of the taking lens is set at a reference lens position or an initial lens position such that an object at an infinite distance may be properly focussed on an imaging plane wherein a photographic film is located. As a practical matter, the object to be focussed is placed at a sufficiently great finite distance, for example 10 m, instead of the infinite distance.
Focus adjustment for a variable focal length camera is provided for two modes comprising telephotography and wide angle photography. For focus adjustment for wide angle photography, the main lens system is moved to a wide angle photography lens position by a focal length changeover mechanism, and thereafter the position of the main lens system is finely adjusted so that an image of an object at a predetermined distance such as 10 m may be formed most sharply on the imaging plane. In order to adjust the focus for telephotography, the main lens system is next moved to a telephotography lens position by the focal length changeover mechanism and the auxiliary lens system is inserted into the photo-taking optical axis. Then, a lens position of the auxiliary lens system is finely adjusted so as to achieve a sharp focus.
As disclosed for example in U.S. Pat. No. 4,307,950 and U.S. Pat. No. 4,725,864, a conventional focus adjusting device uses an adjusting screw against which a lens holder of an auxiliary lens system abuts, under the biassing force of a spring. The lens holder is fixedly mounted on a rotary shaft which is supported at spaced fixed sections rotatably and movably in the axial direction. By rotating the adjusting screw by means of a screwdriver, the lens holder as well as its rotary shaft is moved in the photo-taking optical axis direction against the spring force thereby finely t adjust the position of the auxiliary lens system. However, when the screwdriver is applied to the adjusting screw and is pushed so as to start rotating it, the lens holder is moved parallel to the photo-taking optical axis by an amount corresponding to the backlash of the screw. When the screwdriver is detached from the screw after completion of focus adjustment, the adjusting screw, now free from the screwdriver, will move backward by an amount corresponding to the backlash by the action of the lens holder which is biassed by the spring. The reference lens position of the auxiliary lens system is therefore displaced by an amount corresponding to the backlash. Adjustment work must therefore be performed having regard for such displacement, and of course this requires skilled technique.